1. Field of the Invention
The present invention relates to a rewritable optical recording medium, in particular, an optical recording medium according to a surface-recording/reproducing system wherein laser light is not irradiated to a recording layer through a substrate but laser light is irradiated for recording/reproducing to the recording layer directly without passing through the substrate, or an optical recording medium according to a near-field optical recording system wherein recording/reproducing are conducted by laser light with use of a flying optical head.
2. Discussion of Background
An optical recording medium is a portable recording medium permitting large volume/high density recording, and there have been rapidly increasing demands of rewritable media for recording dynamic images or a large volume file for computers in current fashion of multi-media. In a magneto-optical recording medium as a kind of such optical recording medium, multi-layers including a recording layer are generally formed on a transparent circular disk-like substrate made of a plastic material or the like; recording and erasing are conducted by irradiating laser light while a magnetic field is applied, and reproducing is conducted by reflected laser light. As recording systems, a so-called optical modulation recording wherein information is erased by applying a fixed magnetic field, and then, recording is conducted by applying a fixed magnetic field having an opposite direction, has conventionally been utilized mainly. In recent years, however, a magnetic field modulation system wherein a magnetic field is modulated according to a recording pattern while laser is irradiated, has been noted as a system in which recording can be made in a single turn (direct overlight) and correct recording can be performed even at a high recording density.
Conventionally, laser was irradiated to the recording layer through the substrate in order to perform recording/reproducing. Recently, a so-called near-field optical recording wherein recording/reproducing are performed by bringing the optical head close to the recording layer, has been noted as means for increasing recording density (Appl. Phys. Lett. 68, p.141 (1996)). This recording method allows reproducing with shorter marks than the limit in the conventional recording (˜λ/2NA: NA is a numerical aperture of an objective lens), which is determined by the wavelength (λ) of laser from a light source by using a solid immersion lens (Hereinbelow, referred to as SIL) head so that the laser beam spot size is reduced, and therefore, recording/reproducing can be realized at a super-high recording density.
In such near-field optical recording, it is necessary to bring the optical head close to the recording medium (˜100 nm). Accordingly, the method of irradiating laser beams directly to the recording layer without passing through the substrate is used unlike the conventional magneto-optical recording medium wherein laser beams are irradiated to the recording layer through the substrate. Namely, the structure for the recording layer in the conventional optical recording medium generally comprises substrate/first protective layer/recording layer/second protective layer/reflective layer. On the other hand, the layer structure of the near-field optical recording has a contrary structure of, for example, substrate/reflective layer/first protective layer/recording layer/second protective layer so that laser beams are irradiated from a front surface side of layer to perform recording/reproducing (a surface-recording/reproducing system). In this case, a flying slider head (a flying optical head) is often utilized in order to bring the recording layer close to the SIL head. Further, in recording, it is reported that magnetic field modulation recording wherein laser beams are irradiated to increase temperature in the recording layer to the Curie temperature or more while a magnetic field is modulated by a thin film coil formed on the slider head, is suitable for the near-field magneto-optical recording.
In the near-field optical recording/reproducing system, the distance between the flying optical head and the recording medium is very small. Accordingly, there are risks that the intensity of recording/reproducing signals is fluctuated to cause a trouble in recording/reproducing even when the distance between the head and the surface of the recording medium slightly changed, or the head and the recording medium are broken by the contact of the head to the recording medium.
The near-field optical recording medium has a land and a groove for tracking laser beams and a header having a format information. Accordingly, the substrate for the near-field optical recording medium is generally obtained by injection-molding a thermoplastic resin by using stampers in which the land, the groove and the header are formed.
Further, for the purpose that an optical recording medium is supported by the spindle of a drive when the medium is set in the drive, a hub is attached to a substantially central portion of a surface of the optical recording medium. In order to keep the distance between the head flying above the optical recording medium and the surface of the recording medium constant, it is important to strictly control a deflection, a curve and so on, i.e., mechanical characteristics of the optical recording medium.
However, since the mechanical strength of a substrate made of a thermoplastic resin is lower than a substrate made of metal or glass, the former is easily deflected. Further, a strain is easily caused in the optical recording medium at an inner peripheral side of the recording/reproducing area in the vicinity of the hub when it is attached to the optical recording medium. Therefore, in the conventional near-field optical recording medium using the substrate formed by injection-molding a thermoplastic resin, it was difficult to keep the distance between the head and the surface of the recording medium constant in the entire recording/reproducing area, and it was difficult to obtain uniform recording/reproducing signals.
Further, it is important that the recording medium has sufficient flying characteristics in the near-field optical recording since the distance between the SIL head and the recording medium is very small. Therefore, if foreign matters having a height exceeding a flying height of the SIL head deposit on the surface of the recording medium, there causes a trouble in recording/reproducing, and in a worst case, crushing by the SIL head may result. In order to avoid this, a method of removing foreign matters depositing on the surface of the recording medium by using the flying head, is proposed. According to this method, the head is floated above the recording medium to scan its surface whereby the foreign matters are sweeped by a head edge of the flying head so that the foreign matters deposited on the recording/reproducing area are moved to the outside of the area. Thus, the cleaning of the surface of the recording medium can be performed. In this method, however, when recording/reproducing operations are repeated, foreign matters collected to an inner side from the recording/reproducing area are again brought to the recording/reproducing area during the rotation of the recording medium, or foreign matters collected to an outer side from the recording/reproducing area are again brought to the recording/reproducing area by head-loading. Thus, the conventional near-field optical recording medium according to the recording/reproducing system using the flying slider head could not maintain cleanness of the recording/reproducing area in a repeated use, and there was a problem in durability and reliability.